Method of alkylating amines



United States Patent 07' Claims priority, application Germany June 1, 1954 3 Claims. (Cl. 260-577) This invention relates to a new and useful method of alkylating amines and is a continuation-impart of copending application Ser. No. 512,102, filed May 31, 1955, now abandoned.

According to conventional methods, the alkylation of amines is effected by transforming substances having alkyl groups, e.g. alcohols and amines, at higher temperature in the presence of catalysts, e.g. hydrochloric acid, phosphorous trichloride and oxylchloride. The required reaction temperature is between 180 and 220 C. In view of this temperature and the low boiling point of one of the reaction components, i.e. the alcohol, the transformation takes place in a closed vessel. Owing to the low boiling point of the alcohol this results in pressures of to 60 atmospheres gauge pressure, depending on the type of the alcohol. 7

The resulting product is a mixture, eig. of ethyl aniline, diethyl aniline, alcohol and ether. In general, the ratio of ethyl aniline to diethyl aniline is 1:3 to 1:5. his very difiicult in this reaction to control the transformation in such a way that only monoethyl aniline or only diethyl aniline is obtained.

It has also been proposed to react highly negatively substituted aromatic amines with tertiary alcohols in the presence of phosphoric acid. In this process, the arcmatic amines are substituted with cyano-, nitroor carboxyl groups in the ortho or para positions and these highly negativating substitutents loosen the hydrogen atoms on the amino nitrogen. Furthermore, highly reactable tertiary alcohols are used and the reaction is capable of being carried out in the temperature range of between and 100 C. This process, however, is not applicable to the transformation of aromatic amines, the reactivity of which is not influenced by highly negativating substituents, with primary alcohols, i.e. alcohols which are relatively slow to react.

The process of the invention relates to the alkylation of mononuclear aromatic amines bearing at least one replaceable hydroegn atom on the amino nitrogen with primary alkanols. T he process of the invention operates at substantially atmospheric pressure and permits a fully mononuclear aromatic amines with no interfering group and having on their amino nitrogen at least one replaceable hydrogen atom are introduced together with an alkylating agent into concentrated phosphoric acid which ismaintained at practically atmospheric pressure and at a temperature which ranges above the boiling point of the amine to be converted at the reaction pressure but below about 235 C. The alkylating agents used are lower primary alcohols or ethers thereof. p

The compounds to be reacted may be introduced in the liquid or gaseous phase into the concentrated phosphoric acid. Due to the temperature of the phosphoric acid, any liquid portions which may be present are evaporated and, therefore, the vapors of the compounds to be reacted, in form of a mixture, pass commonly upwardly through the phosphoric acid as a stream of gas; bubbles. The vaporous compounds leave the catalyst bed of phosphoric acid on top thereof and may there be separated from the catalyst without any difiiculty. In this manner, a continuous transformation is possible by introducing the compounds to be reacted continuously into phosphoric acid heated at high temperatures and withdrawing the reaction products on top'of this acid after having passed through the catalyst bed.

It is preferred in carrying out the process to use the phosphoric acid in form of reaction columns which may be several meters in length, e.g. 4 to 8 meters or more. However, shorter reaction columns may be used depending upon the efficiency desired of the conversion. Thus, it has been found, for example, that a considerable reaction will also occur with a phosphoric acid column having a total height of as little as 5 centimeters so that, e.g. with a height of centimeters, the transformation. may be substantially completed after two passages only. For full utilization of the existing catalyst column, it is. preferred to introduce the compounds tobe converted from below or into the lower section of thiscatalyst column. p N

If the compounds to be converted have not yet reacted satisfactorily when being withdrawn at the upper end, e.g., when using a short catalyst column, the re-- action mixture may either be returned into the same. catalyst column or introduced into a second and, if desired, further reaction columns until the conversion has progressed to the extent desired.

By passing the stream of vapor bubbles through the column of catalyst liquid,- a large surface of contact is provided between the compounds to be reacted and the catalyst so that the reaction between the reactants will proceed easily and rapidly within the temperature range to be used in accordance with the'invention. This per--. mits operation without the use of external pressure so that the process may be carried out at practicallyatmospheric pressure. If desired, it is also possible to carry out the process while applying a subatmospheric. pressure to the reaction column. In this way, compounds which are difiicult to volatilize can be successfully con-. verted into the vapor phase Within the temperature range used in accordance with the invention.

Particularly suited for the' process of this invention are aromatic amines having at least one reactive hydrogen atom on the amino nitrogen and boiling below about 220 to 225 C; at atmospheric pressure. Suitable amines are especially aromatic amines such as aniline or anilines' substituted with lower alkyl radicals, e.g. toluidine, xyli'-' dine, ethyl ani-lines and the like. The alkylating age'nts used for the process of the invention are lower primary alcohols or ethers thereof. Examples for these include methanol, ethanol and propanol-l. The lower limit ofthe temperature range used in accordance with the in-.

tion that no decomposition or undesirable side reactions occur. Temperatures up to about 235 C. have been:

found to be particularly suit-able; a temperature of 'between 180 and 220 C. being most preferred forcarry-w ing out the process of the invention. In choosing the compounds tobe reacted, careshoul be taken that the reaction products formed are sufliciently volatile within the temperature range of thephosphoric acid column at the reaction pressure. It is not'absolutely- Fatented July 4, 1961 necessary for the reaction products to be already completely evaporated, it being also possible to remove part of the reaction products from the liquid column by means of the other vaporous portions flowing through. At any rate, the reaction products should be volatile without too much difficulty under the reaction conditions to ensure that they are actually discharged from the reaction column in continuous operation of the process. An attempt made to carry out the process at lower temperatures, e.g. in the range of 80 to 120 C., shows that practically no conversion occurs between the amines and the alkylating agents. Moreover, at these low temperatures, the compounds introduced into the phosphoric acid are, of course, not capable of being removed therefrom in gaseous state so that a continuous conversion as it is possible in accordance with the invention is not capable of being carried out at these low temperatures.

Owing to the specific control of the reaction conditions and of the particular amounts charged of compounds to be reacted the process of the invention offers the possibility of producing either substantially only monoalkylated products and only little dialkylated products or to form substantially dialkylated amines. In addition to the quantity of alcohol fed, reaction conditions essential to this end are the residence time in the catalyst bed, i.e. especially the height of the catalyst column, and the reaction temperature.

Example 1 1000 kgs. of concentrated phosphoric acid are filled into a reaction vessel of, for instance, 1 meter diameter and 300 cms. height and heated, by a suitable heating coil, to a temperature of 180 to 220 C. A mixture of, for instance, 1 part by weight of aniline and 2 parts by weight of methanol is passed through the catalyst liquid at a rate of 100 to 300 liters per hour. After passage through the catalyst the reaction product, consisting of about 92 to 95 percent of dimethyl aniline and methanol of about 75 percent strength, is passed, without cooling, into a fractionating column in which methanol of 98 to 99 percent concentration is separated and the reaction product, i.e., in this case dimethyl aniline and water, is continuously removed from the still. On separation of the water in a separating vessel, there results a dimethyl aniline containing 92 to 95 percent of dimethyl aniline, 7 to 4 percent of monomethyl aniline and l to 2 percent of aniline.

Example 2 The process is carried out as in Example 1 except that the reaction mixture before the separation of methanol is passed once more, with addition of one further part by weight of methanol, through a second, smaller reaction vessel which is also filled with phosphoric acid, thus obtaining a dimethyl aniline with a purity above 99 to 99.5 percent.

Example 3 The process is carried out as in Example 1 except that the aqueous methanol is separated after the first passage, about 1 part by weight of methanol is added to the resultant dimethyl aniline of 90 to 95 percent strength and the mixture is put through a second similar reaction unit or alternately through the first one, thus also obtaining a product of a purity of at least 99 to 99.5 percent.

Example 4 Instead of methanol, dimethyl ether or a mixture of dimethyl ether and methanol is passed through the reactor in the manner described in Example 1, thus obtaining a product containing 90 to 95 percent of dimethyl aniline.

Example 5 The process is carried out as described in Example 1 except that the aniline-methanol mixture fed is heated to 100-200 C. In this way it is achieved that the reaction,

4 i once the catalyst had been heated to about 200 C., proceeds without heating, maintaining the reaction temperature.

Example 6 The alkylation with alcohol to monoand diethyl aniline proceeds in the same way if the process is carried out according to Example 1. It is possible by suitable choice of (a) the temperature and (b) the mixing ratio between aniline and alcohol to determine whether especially monoethyl aniline with small amounts of aniline and diethyl aniline or, on the other hand, diethyl aniline with little mono-ethyl aniline and practically no aniline is to be obtained.

Example 7 7.5 liters of phosphoric acid are filled into a cylindrical tube of 150 cms. in length and 10 cms. in diameter and equipped for electric heating. The temperature is slowly increased to 220 C.

A mixture comprising 1 part by weight of o-toluidinc and 3 parts by weight of ethanol is introduced into the heated phosphoric acid at a rate sufficient to put through 3 kgs. per hour. The efiluent gas mixture is condensed and the dillute alcohol is driven off from the alkylation mixture by simple distillation.

The alkylation mixture consists of:

4 percent of unchanged toluidine, 35 percent of N-monoethyl toluidine, and 61 percent of N-diethyl toluidine.

Example 8 6 liters of phosphoric acid are saturated with 1.5 liters of xylidine in the apparatus and under the conditions described in Example 7. Following this, the procedure described in Example 7, is followed, i.e., a mixture consisting of 1 part by weight of xylidine and 3 parts by weight of methanol is introduced. The effiuent gas mixture is condensed and substantially consists of alkylated xylidine and only a few percent of unchanged xylidine.

Example 9 6 kilograms of phosphoric acid and 6 kilograms of otoluidine are placed into the apparatus described in Example 7. Upon heating to 220230 0, alcohol is introduced into the tube from below. A column mounted on top of the apparatus takes care that only the excess alcohol diluted with water distils. The process is discontinued as desired depending upon the product intended to be recovered, i.e., the desired mono-diethyl toluidine or substantially only diethyl toluidine. The introduction of alcohol is stopped and the diethyl toluidine is distilled off after having shut off the dephelgmator arranged in the column. Fresh toluidine may be immediately introduced into the remaining catalyst and the process may be continued as described above. A yield of 99 to 100 percent of monoor diethyl toluidine is obtained.

In the examples given above it has been found that the catalyst is durable almost without a limit, but at least ensures a 40 to 50-fold conversion or transformation, based on the weight of the catalyst. For the rest, in case of a deterioration of the catalyst, the same can be regenerated in a simple manner and thus be made ready for renewed use.

It will be appreciated that alkylations of any kind can be carried out by the novel method. Practically no waste gases or other decomposition products are formed.

The continuous process hereinbefore described offers a substantial advantage over the known methods, since the reaction products are brought together in their original form in the presence of catalysts, without pressure, and the desired product is obtained in commercial purity, practically without losses of the reactants.

While the invention has been described in detail with respect to certainnow preferred examples and embodiments of the invention it will be understood by those skilled in the art after understanding the invention that various changes and modifications may be made Without departing from the spirit and scope of the invention and it is intended, therefore, to cover all such changes and modifiactions in the appended claims.

I claim:

1. Process for alkylating amines which comprises maint aining a liquid body of concentrated phosphoric acid at a temperature ranging from above the boiling point of the amine to be alkylated to 235 C. and a pressure not substantially in excess of atmospheric, introducing an amine selected from the group consisting of aniline and aniline substituted with a lower alkyl radical at the nucleus and an alkylating agent selected from the group consisting of lower primary alkanols and their lower alkyl ethers into said liquid body to bubble through said body in contact and separating the gaseous reaction products containing the alkylated amine.

body is maintained at a temperature between about 180 and 220 C.

3. A process according to claim 1 in which said liquid body is a body of a height of at least 4 meters and in which said amine and alkylating agent are introduced into the lower portion of said body to bubble upwardly therethrough.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Perry: Chemical Engineers Handbook, 3rd ed., pp.

2. Process according to claim 1 in which said liquid 20 1216-1217, McGraw-Hill, New York (1950). 

1. PROCESS FOR ALKYLATING AMINES WHICH COMPRISES MAINTAINING A LIQUID BODY OF CONCENTRATED PHOSPHORIC ACID AT A TEMPERATURE RANGING FROM ABOVE THE BOILING POINT OF THE AMINE TO BE ALKYLATED TO 235*C. AND A PRESSURE NOT SUBSTANTIALLY IN EXCESS OF ATMOSPHERIC, INTRODUCING AN AMINE SELECTED FROM THE GROUP CONSISTING OF ANILINE AND ANILINE SUBSTITUTED WITH A LOWER ALKYL RADICAL AT THE NUCLEUS AND AN ALKYLATING AGENT SELECTED FROM THE GROUP CONSISTING OF LOWER PRIMARYALKANOLS AND THEIR LOWER ALKYL ETHERS INTO SAID LIQUID BODY TO BUBBLE THROUGH SAID BODY IN CONTACT AND SEPARATING THE GASEOUS REACTION PRODUCTS CONTAINING THE ALKYLATED AMINE. 